As is known, a rotary veneer cutting machine for tree trunks is a machine which, by cutting action exerted by a blade placed laterally and parallel to the axially supported trunk between rotating chucks, removes therefrom a strip of wood of uniform and constant thickness and of a width equal to the length of the trunk.
The strip of wood cut from the trunk is compressed in order to stabilize the trajectory of penetration of the blade in the trunk, between the point of the blade and the point of a bar, termed pressure bar, parallel thereto. The action of the bar and blade in the contact zone thereof generates a force F acting on the trunk with variable direction and intensity during the veneer cutting process.
Said force can be resolved into a horizontal force directed toward the axis of the trunk and due principally to the aforesaid compression action of the rotary-cut veneer and into a vertical force directed upward and due principally to the static and dynamic resistance of the wood to distortion and to sliding on the blade and pressure bar and to the penetration force of the blade, which in turn depends on the physical and geometrical characteristics of the blade and wood and the degree of compression of the rotary-cut veneer.
It is thus clear that, other conditions being equal, the vertical component is highly variable with the peripheral speed of the trunk and decreases therewith. Due to the known principles of mechanics, the force F is equivalent to a force F equal and parallel thereto applied to the rotation axis of the trunk and a moment acting in a direction opposite that of said rotation.
While the moment only generates a torsional stress of the trunk and thus does not change the form and interaction conditions with the blade, the force F, distributed along the axis of the trunk, causes bending stress therein which tends to differentiate the conditions of the veneer cutting process along the length of the blade, causing irregularity in the thickness of the veneer and cracking and undulations generated by the difference in the development of the central part in relation to the side parts.
Moment M, even though it does not change the shape of the trunk, is the cause of another problem, especially for high-speed veneer-cutting machines designed to cut veneer from trunks down to a very small core diameter (on the order of 7-8 centimetres or less) to achieve high volumetric outputs. Given the small final diameter of the trunk the axially gripping jaws must have an equally small diameter and this means that at the beginning of the rotary veneer cutting the concentration of force thereon due to the moment M can reach such high values as to endanger the strength of the wood and consequently the rotating action.
To overcome these imperfections, the known art has placed counteracting supporting rollers acting on the opposite side of the trunk from the blade and on its top, i.e. along the two directions, vertical and horizontal, which make up the force F, so as to counteract the effect thereof, and in addition the top rollers have been motorized to exert an additional rotating action directly on the periphery of the trunk.
This solution however display problems, the first of which is that the vertical component of force F, as mentioned above, decreases progressively with the decrease in the diameter of the trunk and consequently the adherence available on the rotating rollers is highly variable and can become too small to allow correct rotation of the trunk.
It has also been thought to shift the top rollers toward the blade to attempt to at least partially balance force F by the rotating effort of said rollers. But this increases more than 90 degrees the angle between the top and side rollers and thus decreases the effectiveness of bending limitation.
The general object of the present invention is to obviate the above imperfections by providing a veneer-cutting machine with adequate means for always counteracting bending and compensating for the torque regardless of the operating conditions of the machine.